Early work with WC focused upon densifying WC by heating to a temperature of, for example, 2000.degree. Centigrade (.degree.C). The densified material was judged unsuitable for use in applications requiring toughness, such as in cutting tools. The unsuitability stemmed largely from the densified material's excessively brittle character.
Efforts to overcome or offset some of the brittleness led to incorporation of an amount of a metal by admixing powdered metal and WC powder to form a composite and densifying the composite at a temperature above that at which the metal melts. The metal, most frequently an iron group metal (iron, cobalt or nickel), was added to impart some of its ductility to the composite. The densified composites, also known as cemented carbides, cermets and hard metals, have been used extensively for several decades.
Cutler (U.S. Pat. No. 4,828,584) discloses ceramic bodies that are at least 98.5% by volume WC with substantially all grains having an average size of less than 15 .mu.m, preferably less than 10 .mu.m and more preferably less than 5 .mu.m. A grain size range of 0.1 to 5.0 .mu.m is reportedly quite useful. Cutler also discloses preparation of the ceramic bodies by sintering greenware made from WC particles having a diameter of less than 15 .mu.m, preferably less than 5 .mu.m. Grain sizes between 5 and 15 .mu.m provide high toughness and grain sizes between 1 and 3 .mu.m yield higher strength and lower toughness. As the grain size increases, the fracture mode changes from intergranular to transgranular fracture.
Maruyama et al. (U.S. Pat. No. 4,753,678) disclose cemented carbides based upon WC and either vanadium carbide or zirconium nitride as a hard phase and 4 to 20% by weight (wt %) of cobalt as a metal or binder phase.
Eric A. Almond et al., in "Some Characteristics of Very-Fine Grained Hardmetals", Metal Powder Report, Vol 42, No. 7/8, pages 512, 514 and 515 (July/August 1987) teach that binder-phase hard metals experience an asymptotic decrease in fracture toughness as grain size decreases.